1. Field of the Invention
The present invention relates to projection optical systems used in printing circuit patterns during the manufacture of large flat panel displays (FPD), and more particularly, to an optical design form that is relatively compact, provides aberration and magnification correction and facilitates a high FPD production rate.
2. Related Art
The manufacture of a liquid crystal display, or a flat panel display (FPD) involves a manufacturing process that is similar to that used in the integrated circuit (IC) industry where computer chips are produced. An exposure system is used to project an image of a reticle containing a circuit pattern so as to expose a photo resist coated substrate. The actual circuit is created after the exposed substrate is processed using standard microlithographic processes. Depending on the particular FPD design this exposure process may be repeated many times on one substrate using reticles with different circuit designs. When all the exposures and microlithographic processing steps have been completed so the desired circuit pattern has been created, the substrate is integrated with other components to create a flat panel display screen.
Although FPDs have been in production since the late 1980s, the current size requirement are for FPDs of up to 42 inches diagonal, with 54 and 60 inches diagonal under development. This places severe requirements on the optics used in the projection optical system. Specifically, many existing optical design forms, if scaled up to 42 inch (and larger) FPD manufacturing size, become unreasonably large, especially from an optical manufacturing and packaging perspective.
Two different imaging processes are conventionally used to lithographically print circuits on flat panel display screens. Lithography tools described in U.S. Pat. Nos. 5,625,436, 5,530,516, 4,769,680 and 5,710,619 create the full circuit by stitching together images of small areas of the FPD circuit design. While acceptable quality and cost FPD screens up to 18 inches have been produced using the stitching imaging technique, the errors inherent in stitching have resulted in a very low product yield during manufacturing of larger displays and marginally acceptable quality. Because of the low yield, the production cost for large FPD televisions has been at an unacceptably high level from the FPD manufacturers' marketing perspective. As a result of the residual stitching errors the quality level of FPD televisions has not been that much better than conventional televisions for consumers to justify the high cost of an FPD television.
The major problem encountered in stitching is that the adjacent small images that create the full FPD circuit pattern are not aligned to each other. There are many sources of pattern alignment errors in optical imaging configurations. However, most of the errors are related to the imaging process requiring the use of multiple lens systems and masks. Misalignment errors result in electrical connection problems in the FPD circuit and/or an image on the screen that has visually displeasing discontinuities. No good solutions have been found to completely eliminate the problems associated with stitching. As a result, FPD manufactures prefer full field imaging or scanning systems to any optical design configuration that employs stitching.
Optical designs, operating at 1× or at some other magnification, compatible with producing 42-inch full field scanners, require very large lenses and/or mirrors. To print 42-inch FPDs, the lithography tool must have a minimum slit height of about 525 mm. (Note that while in the United States, screen dimensions are usually specified using the English system, while optical design and tool dimensioning is usually done in metric.) The requirement that the optical design form be telecentric results in at least one element in the optical design being 525 mm in diameter and more, typically at least 1,200 mm for the FPD manufacturers preferred optical design form.
For 1× refractive and catadioptric design forms, a minimum of a dozen and as many as several dozen lenses and mirrors are required. It is extremely difficult and very costly to achieve the optical performance required for printing 42 inch and larger FPD circuits using optical designs with refractive elements due to chromatic aberration in the refractive elements and problems with index of refraction homogeneity in the lens material. Even if an optical design is optimized to minimize chromatic aberration, there is a practical limit on the usable spectral bandwidth of the source. The reduced usable spectral bandwidth results in less available light to expose the photoresist. As a result, refractive and catadioptric manufacturing tools produce fewer FPDs per hour then optical design forms that do not suffer from chromatic aberration. The lower production rate results in higher costs for FPD televisions.
In addition to cost disadvantages, the image quality and distortion of refractive and most catadioptric design forms are compromised by the lens material's inhomogeneity, which degrades image quality and introduces distortion. While lens material inhomogeneity errors can be partially compensated for during glass production, or in the lens optical fabrication process, both methods add significant costs. In optical design, a system optimized to minimize the physical size of the lenses has many elements, while a design optimized to have just a few elements will have very large elements. However, in either of those scenarios, the total thickness of glass required in both designs will result in unacceptable inhomogeneity-related problems. As a result, optical design forms for tools to produce FPDs of approximately 24 inches and larger, which make extensive use of refractive materials, are extremely expensive to manufacture or cannot be built because the quality level glasses are not readily available.
Reflective optical design forms operating at 1× magnification are successfully being used in the microlithography industry, including the production of FPDs. An ASML Micralign design form first described in U.S. Pat. No. 3,748,015, which is shown in FIG. 1A, has been used in manufacturing of 32 inch FPD. The design in FIG. 1A has two spherical mirrors, a primary concave mirror 101, and a secondary convex mirror 102. Note that, as shown in FIG. 1A, the primary mirror is used as a reflector twice. A reticle 103, that has the desired FPD circuit pattern drawn on it, is positioned off axis with respect to the optical system's optical axis. The image of the reticle 103 is projected onto a substrate 104 located symmetrically on the opposite side of the optical axis as the reticle. In this design form, the two spherical mirrors are configured to have good image quality and low distortion over an annular field (an annular field optical design concept is described in U.S. Pat. No. 3,821,763). Aberrations in this design are corrected by using concentric optical surfaces, selecting the surface radii of curvatures with specific relationships and having the reticle and the FPD substrate symmetrically arranged relative to the optical system. Specifically, the radius of curvature of the convex mirror is one-half of the radius of curvature of the concave mirror. (Note that it is also possible to use two convex and one concave mirrors instead of two concave and one convex, but such a design is considerably more difficult.)
Using these optical design principles results in the optical system being naturally corrected for the aberrations spherical, coma and distortion. There can be a substantial amount of astigmatism with this design form, with the amount dependent on the reticle size and the numerical aperture at which the system is operating. The ability to correct the astigmatism is extremely important because for this optical design form the astigmatism is what limits the usable slit width, which in turn determines the production rate for the FPD substrates. Astigmatism can be corrected to a limited extent by small deviations of the surfaces from concentricity, or by a small change in the convex mirror radius of curvature.
While this design form is capable of meeting the optical performance requirements for printing 32 inch FPDs, the image quality and distortion is marginal at best when compared to the particular resolution and overlay requirements typically needed for a 42 inch display. Because of the large increase in astigmatism when imaging a 42 inch display, the usable slit width is unacceptably small from the product production rate perspective, which will be discussed in the following sections. Also, scaling the 32 inch design to print 42 inch FPD results in one of the mirrors in the optical system being a minimum of 1.2 meters in diameter. This size mirror presents optical manufacturing challenges and very difficult packaging problems both from it's physical size perspective and its approximately 700 kg weight. Also, with this size and weight optic, problems with mounting, alignment, and gravity-induced distortions are generally encountered. In comparison, the largest mirror in the tool used to manufacture 32-inch FPD is about 800 mm in diameter and weighs on the order of 200 kg.
In the two-mirror approach of FIG. 1A, an arcuate-shaped region on the reticle is formed on the FPD substrate, as illustrated in FIG. 1B. In FIG. 1B, A is sable slit width. Optical performance is acceptable at any point within the slit width A. B is the center of slit. Optical performance gets worse on either side of the center line B. The rate of performance falloff generally increases exponentially with increased distance from the slit center line B. C is the slit height. An FPD has a diagonal dimension from 42 to 60 inches. Aspect ratio (length to width) is generally the industry standard of 16:9. A larger FPD diagonal dimension results in an increased slit height. Slit heights range for 550 mm for a 42 inch diagonal to 775 mm for a 60 inch diagonal. In operation the slit is scanned from left to right (or vice-versa) to print the FPD circuit pattern. At any point in time only the area defined by the slit is being exposed on the FPD.
To image the full circuit pattern of the FPD substrate, the arcuate-shaped field of view is scanned across the full width of the reticle. This creates an image of the circuit pattern on the photoresist coated substrate. The height of the arcuate shape is designed to be the same as the vertical axis of the FPD screen, which for a 42-inch screen is about 525 mm. This enables a circuit pattern to be imaged on the substrate in a single scan. For a 42 inch FPD the screen width is about 930 mm.
The width of the arcuate surface depends on the residual aberrations of the optimized optical design. To achieve a high production rate, a large slit width is desired. Larger slit widths result in more photons reaching the photoresist per unit time. A greater number of photons per unit time enables a shorter exposure time when printing the circuit pattern, thus enabling more FPD substrates to be printed per hour. Based on the typical power level in a FPD tool source system, an arcuate width of at least 5 mm is desired.
It is important to note that as the FPD size increases, the residual aberrations increase in any 1× optical system. Not only does the magnitude of the residual aberrations increase with FPD size, but aberrations which could be previously ignored because of their small size now reach a magnitude where they must be considered in the optical design optimization process. The aberration increase is not linear with FPD size. The aberrations of concern increase with square, fourth and sixth power of the FPD size.
In order to meet the image quality and distortion requirements necessary to print FPD circuit patterns, these aberrations must be reduced in magnitude to an acceptable level. Adjusting the various parameters, such as the optical surface curvatures, surface shape, optical element spacing, aperture stop location, etc. can control the magnitude of the aberrations. For those familiar in the art of optical design, it is a well known principle that the number of different aberrations that can be corrected is directly related to the number of parameters, or degrees of freedom, that are available for adjustment. For example, six degrees of freedom enable six aberrations to be corrected (“corrected” means that the magnitude of an aberration can be improved). However, these same degrees of freedom are needed to control other aspects of the optical design, such as the first order design characteristics magnification, focal length, back working distance, etc. As a result, after accounting for the degrees of freedom needed to control the first order design characteristics, only one or two of the degrees of freedom out of the original six may be available to correct aberrations. Unfortunately, some of the optical design variables that are considered “degrees of freedom” have very little impact on the relative magnitude of the aberrations. While an optical system may have six degrees of freedom, only four of those variables may impact the aberrations' magnitudes in a meaningful way. Because of this the number of degrees of freedom is therefore an important factor in an optical design form. From the aberration correction perspective, an optical design form that has many degrees of freedom will enable superior optical performance to be achieved as compared to a design form that only has a few degrees of freedom.
In designing the conventional two-mirror system described in U.S. Pat. No. 3,748,015, there are only 9 degrees of freedom: spacing between object plane and concave mirror 101; radii of curvatures of concave mirror 101 and convex mirror 102; x and y tilt of concave mirror 101; concave mirror 101 to convex mirror 102 distance; x and y tilt of convex mirror 102; and concave mirror 101 to image plane distance. (For spherical mirrors, a lateral displacement, or decenter, is the equivalent to tilting the surface.) During the optical design process, six of these degrees of freedom are needed to control numerical aperture, magnification, focal length and alignment related errors. This leaves three variables, which is insufficient to control all the aberrations plus any other optical performance considerations, such as telecentricity.
Accordingly, for the manufacture of large-scale FPD's, it is desirable to have an optical design form that enables relatively small optics to be used in the design and to have sufficient degrees of freedom available to correct all the critical aberrations and related optical performance considerations.